The present invention relates to novel methods and materials for minimizing metal-oxide deposits on steam generator tubes in the secondary side of pressurized nuclear steam generators by utilizing specific high-purity polymer dispersants.
At present, no method or process exists for eliminating and preventing the deposit of metal-oxides/sludge in the secondary side of nuclear steam generators during operation of the generator. The only method and process existing for controlling the amount of impurities that enter into the secondary side of the steam generator is the utilization of pure water. The consequences resulting from the buildup of metal oxides within the secondary side of a steam generator are reduced steam output thereby resulting in lost electrical output from the generating plant, increased water level fluctuations within the steam generator thereby resulting in lower steam and electrical output, and the initiation of corrosion deposits within the heat exchanger through the concentration of the dissolved chemical species from the secondary water. The corrosion within the secondary side of a pressurized nuclear steam generator ultimately may result in tube plugging and sleeving and the eventual loss of electrical output because of lost heat transfer or flow imbalances unless the steam generators themselves are replaced at a cost of approximately $200,000,000 per plant.
Accordingly, all known processes for eliminating deposits of metal oxides in the secondary side of recirculating steam generators have been directed to the removal of these deposits after they build-up in the heat exchanger. The major technique utilized for the removal of suspended and dissolved impurities from the secondary side of the recirculating steam generator involves removing a portion of the water from the steam generator during operation on a continuous or periodic basis through a blowdown system. Typically, the blowdown system only removes up to 10 percent of the total metal oxides or impurities which enter the recirculating nuclear system generator during operation, with the remaining metal oxides or impurities continuing to build-up and to be deposited within the secondary side of the recirculating nuclear steam generator. This deposition may result in pressure loss, level fluctuations, and corrosion of the secondary side of the nuclear steam generator.
Several mechanical and chemical methods have been suggested for removing metal oxides or impurities from within the secondary side of nuclear steam generators when the system is near or at shutdown conditions. One of these methods utilizes sludge lancing at shutdown which employs high pressure water to flush loosely adhered oxide deposits and sludge from the lower tube sheet of the nuclear steam generator. This process typically does not address deposition of corrosion in the upper tube support plates and does not clean any clogged crevices on the secondary side of the nuclear steam generator. The percentage of metal oxides or corrosion removed by this process is about two percent of the total oxides entering the nuclear steam generators over a typical 18-month fuel cycle. The cost of completing a sludge lancing is approximately $350,000 for each 18-month fuel cycle in a typical four-loop plant.
Another method suggested for removing metal oxides/sludge at shutdown from the secondary side of a nuclear steam generator is the bundle-flush process. This process entails directing flush water from the upper part of the recirculating nuclear steam generator to remove the loose sludge from the upper tube support plates. The cost of the bundle flush process is approximately $500,000 per application; however, the process only removes the soft, loosely adhered sludge, and does not remove sludge which is strongly adhered to the heat transfer surfaces. Additionally, the small crevices within the heat transfer structure are not cleaned at all by this process. Accordingly, this process is of limited value and does not overcome the problem of removing strongly adhered deposits or impediments within the heat-transfer structure.
Crevice flush techniques have been suggested in an attempt to open or clean closed or packed crevices by heating the secondary side of the nuclear steam generator above a boiling point with an inert atmosphere overpressure and then releasing this overpressure. The crevice flush process results in a boiling action which purportedly flushes the impurities from the crevices in the nuclear steamed generator. However, this method has only demonstrated limited effectiveness and is very time consuming, thereby prolonging downtime, an added cost in the electrical industry.
Chemical-soak techniques have been suggested for use during shutdown to promote removal of loose sludge and loosely adhered deposits within the nuclear steam generator. The chemical soaks employ amines such as dimethylamine and morpholine. These soaks have exhibited limited effectiveness in removing loosely adhered deposits, and the amount or percentage of metal oxides removed is less than acceptable. The advantage of this process is that the cost is low; but the disadvantages of this method are that the process is time consuming, and the effectiveness and the amount of metal oxides removed is less than satisfactory.
Pressure-pulse cleaning or water slapping are mechanical methods which are utilized during an outage or shutdown for removing loosely adhered sludge from the upper tubes or the tube support plates of the nuclear steam generator. The sludge or deposits are removed by raising the water on the secondary side to a desired level and then injecting a high pressure gas such as nitrogen into the water. The bursting of the bubbles as the gas approaches the surface of the water partially removes limited amounts of the loosely adhered sludge or oxide deposits. This technique may increase the amount of metal oxides removed from 5-15 percent of the total amount of metal oxides deposited within the nuclear steam generator; however, this method does not remove hard deposits and does not open crevices packed with metal oxides or other corrosion. The cost of a pressure pulse cleaning is typically $200,000 to $600,000 per unit. It is recommended that such a cleaning be employed every one-to-four refueling cycles.
Finally, the methods of chemical cleaning at low or high temperatures and the use of chemically enhanced pressure pulse cleaning are processes utilizing specific organic materials that dissolve the metal-oxide deposits within the nuclear steam generator. The cleaning solution dissolves the metal-oxide deposits, and the spent cleaning solution must be processed and properly disposed. The chemical-cleaning processes may be selected to remove specific metal oxides contained within the nuclear steam generator. Variations of the chemical cleaning process include the heating of the cleaning solution above the liquid-boiling temperature under an inert atmosphere and then releasing the pressure to force boiling in the cracks and the crevices and the use of pulse-cleaning techniques to promote circulation and movement of the cleaning solution. The chemical cleaning processes remove virtually one-hundred percent of the metal-oxide deposits within the secondary side of the recirculating steam generator, but at a cost of between $5,000,000-$10,000,000 per cleaning. Many of the nuclear generating plants in operation may require chemical cleaning at least once during their lifetime.
Thus, each of the mechanical prior art methods for removing metal-oxides from the secondary side of the nuclear steam generator is directed to removing the loosely deposited oxides within the heat-exchange structure that results from the continued operation of the nuclear power plant. Although chemical cleaning removes substantially all metal oxides, such a process is extremely expensive and time consuming. Accordingly, none of the known chemical or mechanical methods is directed to preventing the deposition or formation of sludge within the secondary side of a nuclear steam generator during operation of the generator. These processes attempt to remove the oxide and corrosive deposits after they have been deposited in the secondary side of the nuclear steam generator, processes which are extremely costly and which result in significant downtime of the nuclear power plant.
Natural polymer dispersants have been used to minimize deposition of sludge deposits in fossil steam generators since the early 1900's, and synthetic polymers have been recently utilized for metal-oxide dispersing and sludge conditioning in fossil steam generators. However, such synthetic polymers have not been qualified for use in minimizing metal-oxide deposition on the secondary side of recirculating nuclear steam generators. Most synthetic polymers developed and used today in water-treatment applications are manufactured using inorganics, such as sodium persulfate, as the initiators of polymerization, and other inorganics as chain transfer agents. However, the sodium and persulfate inorganics contribute unwanted contaminants in significant excess to those required for application in nuclear steam generator units. Polymers typically used in boilers contain inorganic solids at concentrations up to 500 times the allowable levels for application to nuclear steam generator units. Inorganic impurities can include sodium, potassium, chlorine, sulfur, fluorine, and phosphorus--elements which are particularly objectionable and damaging when used in nuclear steam generator operations.
Synthetic polymers used in water treatment applications are typically neutralized with sodium or potassium, forming the inorganic salt. Although ammonia-neutralized versions have been used to a small extent, ammonia is a known copper-alloy corrodent. Polymer neutralization minimizes system upset potential. Polymers have been used unneutralized, but the feed-rate variations have been known to cause system upsets by lowering the pH, thereby resulting in corrosion to the operating system.